Field of the Invention
The present invention relates to a substrate holder for use in a polishing apparatus for polishing a substrate, such as a wafer, and more particularly to a substrate holder for holding a substrate and pressing the substrate against a polishing surface. The present invention further relates to a polishing apparatus and a polishing method using such a substrate holder.
Description of the Related Art
With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer with use of a polishing apparatus by placing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
The polishing apparatus of this type has a polishing table that supports the polishing pad, and a substrate holder for holding the wafer. The substrate holder is often called a top ring or a polishing head. This polishing apparatus polishes the wafer as follows. The substrate holder holds the wafer and presses it against the polishing surface of the polishing pad at predetermined pressure. The polishing table and the substrate holder are moved relative to each other to bring the wafer into sliding contact with the polishing surface to thereby polish a surface of the wafer to flat and mirror finish.
When polishing the wafer, if a relative pressing force applied between the wafer and the polishing pad is not uniform over the surface of the wafer in its entirety, lack of polishing or excessive polishing would occur depending on the pressing force applied to each portion of the wafer. Thus, in order to even the pressing force exerted on the wafer, the substrate holder has at its lower part a pressure chamber formed by a flexible membrane. This pressure chamber is supplied with fluid, such as air, to press the wafer through the flexible membrane by the fluid pressure.
However, since the above-described polishing pad has elasticity, the pressing force becomes non-uniform in an edge portion (peripheral portion) of the wafer during polishing. Such non-uniform pressing force would result in so-called “rounded edge” which is excessive polishing that occurs only in the edge portion of the wafer. In order to prevent such rounded edge, the substrate holder has a retaining ring for retaining the edge portion of the wafer. This retaining ring is configured to be vertically movable relative to a top ring body (or carrier head body) and press a region in the polishing surface of the polishing pad around the wafer.
There has recently been an increasing demand for controlling a polishing profile in the edge portion and its neighboring portion of the wafer. In order to meet such a demand, there has been proposed a substrate holder having two retaining rings with different diameters disposed around the wafer. For example, Japanese laid-open patent publication No. 2008-302464 discloses a substrate holder having a first retaining ring and a second retaining ring which are configured to be able to control their pressing forces independently of each other, so that uniformity of the polishing profile is improved.
The inventors of the present invention have found from results of various experiments that, during polishing, the wafer is pushed against an inner surface of the retaining ring by a frictional force generated between the wafer and the polishing surface and that a downstream portion, with respect to a rotating direction of the polishing pad, of the wafer is polished at a very high polishing rate, compared with a central portion of the wafer. Further, the inventors of the present invention have found the fact that, under such wafer polishing conditions, there are cases where the polishing profile cannot be controlled in the wafer edge portion and as a result a desired polishing profile cannot be obtained even when using the two retaining rings with different diameters.
During polishing, the frictional force is produced between the wafer and the polishing pad. This frictional force acts as a lateral force (or a horizontal force) on the retaining ring. Japanese laid-open patent publication No. 2007-268654 (hereinafter, this publication will be referred to as patent document) discloses a substrate holder designed to support this lateral force by a retaining ring guide arranged around the retaining ring. However, in this substrate holder, a point at which the retaining ring guide supports the retaining ring is located away from the polishing surface. This arrangement causes the retaining ring to tilt around this supporting point when it is receiving the lateral force from the wafer. As a result, the retaining ring cannot apply a desired pressing force to the polishing surface uniformly. In addition, the retaining ring may be deformed in some portions thereof by the lateral force applied from the wafer. Such a deformed portion prevents the retaining ring from exerting the desired pressing force on the polishing surface.
Moreover, the substrate holder disclosed in the above-mentioned patent document has a problem that sliding contact between an outer surface of the retaining ring and an inner surface of the retaining ring guide produces wear particles. If the wear particles fall onto the polishing surface, defect of the wafer could occur. Thus, in order to prevent the wear particles from falling onto the polishing surface, a flexible sheet is provided in the substrate holder. However, if the substrate holder is modified so as to lower the supporting point of the retaining ring (i.e., the contact point between the retaining ring and the retaining ring guide) for the purpose of reducing the tilting movement of the retaining ring, the flexible sheet cannot be installed in the substrate holder. As a result, the wear particles would fall onto the polishing surface.
Japanese laid-open patent publication No. 2009-190191 discloses a substrate holder that does not have the retaining ring guide as disclosed in the above patent document. Instead, a spherical bearing is provided above the center of the wafer so as to support the lateral force applied from the wafer to the retaining ring. This configuration does not produce the wear particles outside of the retaining ring and therefore the wear particles do not fall onto the polishing surface.
However, the above spherical bearing is located away from the polishing surface. This arrangement causes the retaining ring to tilt around this spherical bearing when it is receiving the lateral force from the wafer. As a result, the retaining ring cannot apply a desired pressing force to the polishing surface uniformly. In addition, the retaining ring may be defaulted in some portions thereof by the lateral force applied from the wafer. This leads to a problem that the deformed portion prevents the retaining ring from exerting the desired pressing force on the polishing surface.